The present invention relates to compositions which are suitable, inter alia, for electrochemical cells with electrolytes containing lithium ions; to their use, for example, in and as solid electrolytes, separators and electrodes; to solid electrolytes, separators, electrodes, sensors, electrochromic windows, displays, capacitors and ion-conducting films which contain a composition of this type, and to electrochemical cells containing such solid electrolytes, separators and/or electrodes.
Electrochemical, in particular rechargeable cells are known in general terms, for example from xe2x80x9cUllmann""s Encyclopedia of Industrial Chemistryxe2x80x9d, 5th Edn., Vol. A3, VCH Verlagsgesellschaft mbH, Weinheim, 1985, pages 343-397.
Of these cells, Aithium batteries and lithium ion batteries occupy a special position, in particular as secondary cells, owing to their high specific energy storage density.
Cells of this type contain in the cathode, as described, inter alia, in the above passage from xe2x80x9cUllnannxe2x80x9d, lithiated manganese, cobalt, vanadium or nickel mixed oxides, as, in the stoichiometrically simplest case, can be described as LiMn2O4, LiCoO2, LiV2O5 or LiNiO2.
These mixed oxides react reversibly with compounds which can incorporate lithium ions into their lattice, for example graphite, with removal of lithium ions from the crystal lattice, where the metal ions such as manganese, cobalt or nickel ions, can be oxidized in the latter. This reaction can be utilized in an electrochemical cell for current storage by separating the compound which takes up the lithium ions, ie. the anode material, and the lithium-containing mixed oxide, ie. the cathode material, by an electrolyte, through which the lithium ions migrate from the mixed oxide into the anode material (charging operation).
The compounds which are suitable for reversible storage of lithium ions are usually fixed lo drain electrodes by means of a binder.
During charging off the cell, electrons flow through an external voltage source and lithium cations flow through the electrolyte to the anode material. During use of the cell, the lithium cations flow through the electrolyte, whereas the electrons flow through a working resistance from the anode material to the cathode material.
In order to avoid a short-circuit within the electrochemical cell, an electrically insulating, but lithium cation-permeable layer is located between the two electrodes. This can be a so-called solid electrolyte or a conventional separator.
Solid electrolytes and separators consist, as is known, of a support material in which a lithium cation-containing compound which is capable of dissociation is incorporated in order to increase the lithium ion conductivity, and further additives, such as solvents, are usually also incorporated.
U.S. Pat. Nos. 5,296,3181 and 5,429,891, for example, propose a copolymer of vinylidene fluoride and hexafluoropropene as support material. However, the use of high-resistance (co)copolymers of this type is afflicted with a number of disadvantages.
Polymers of this type are not only expensive, but can also only be dissolved with difficulty. Furthermore, owing to their comparatively low lithium cation conductivity, thy increase the resistance of the cell, and consequently the electrolyte, which usually consists of a lithium cation-containing compound, such as LiPF6, LiAsF6 or LiSbF6, and an organic solvent, such as ethylene carbonate or propylene carbonate, has already been added to the insulating layer during production (U.S. Pat. Nos. 5,296,318 and 5,429,891). In addition, polymers of this type can only by processed with, for example, high proportions of plasticizers, for example di-n-butyl phthalate, and pyrogenic silicas, which are added in order firstly to ensure adequate film-formation and cohesion of the electrolyte layer and bondability to the electrode layers and secondly ensure adequate conductivity and permeability for lithium cations. The plasticizer must, before the batteries are used, be separated quantitatively from the laminate comprising anode, solid electrolyte or separator layer and cathode layer in an extraction step which is expensive and extremely difficult on an industrial scale.
WO 97/37397 relates inter alia, to a mixture Ia comprising a mixture IIa consisting of
a) from 1 to 95% by weight of a solid III, preferably a basic solid III having a primary particle size of from 5 nm to 20 xcexcm, and
b) from 5 to 99% by weight of a polymeric composition IV obtainable by polymerization of
b1) from 5 to 100% by weight, based on the composition IV, of a condensation product V of
a) at least one compound VI which is capable of reacting with a carboxylic acid or a sulfonic acid or a derivative or a mixture of two or more thereof, and
b) at least 1 mol per mole of the compound VI, of a carboxylic acid or sulfonic acid VII containing at least one free-radical-polymerizable functional group, or of a derivative thereof or of a mixture of two or more thereof, and
b2) from 0 to 95% by weight, based on the composition IV, of a further compound VIII having a mean molecular weight (number average) of at least 5000 containing polyether segments in the main or side chain,
where the proportion by weight of the mixture Ia in the mixture Ia is from 1 to 100% by weight.
Although the systems described therein already have excellent properties, in particular when used in electrochemical cells, such as, for example, excellent short-circuit resistance high mechanical stability and good processing properties, use of these systems usually requires that the actual foil production or photocrosslinking step in the production of, for example, cast foils, be carried out under inert-gas conditions.
A further improved system for use in electrochemical cells, in particular a composition which can be processed better, ie. with avoidance of inert-gas conditions, is described in DE-A 198 19 752. This relates to a composition comprising:
(a) from 1 to 99% by weight of a pigment (I) having a primary particle size of from 5 nm to 100 xcexcm which is a solid Ia or a compound Ib which acts as cathode material in electrochemical cells or a compound Ic which acts as anode material in electrochemical cells or a mixture of the solid Ia with the compound Ib or the compound Ic,
(b) from 1 to 99% by weight of a polymeric material (II) which comprises:
(IIa)from 1 to 100% by weight of a polymer or copolymer (IIa) containing reactive groups (RG) on the chain in terminal and/or lateral positions which are capable of crosslinking reactions in the presence of heat and/or with UV radiation, and
(IIb) from 0 to 99% by weight of at least one polymer or copolymer (IIb) which contains no reactive groups (RG).
In more detailed investigations, it has now been found that a further-improved composition of the type under discussion here and a highly porous membrane can also be obtained if the pigment content of the composition described in DE-A 198 19 752 is significantly reduced.
Accordingly, the present invention relates to a composition comprising:
(a) from 0 to less than 1% by weight of a pigment (I) having a primary particle size of from 5 nm to 100 xcexcm which is a solid Ia or a compound Ib which acts as cathode material in electrochemical cells or a compound Ic which acts as anode material in electrochemical cells or a mixture of the solid Ia with the compound Ib or the compound Ic,
(b) more than 99 to 100% by weight of a polymeric material (II) which comprises:
(IIa) from 1 to 100% by weight of a polymer or copolymer (IIa) containing reactive groups (RG) on the chain in terminal and/or lateral positions which are capable of crosslinking reactions in the presence of heat and/or with UV radiation, and
(IIb) from 0 to 99% by weight of at least one polymer or copolymer (IIb) which contains no reactive groups (RG).
The novel composition above has the following surprising properties:
Although the pigment only contains a small proportion of a pigment (I), or none at all, the composition is highly active and mechanically stable; it is highly suitable as an ion-conducting polymer electrolyte system, particularly suitable for use in lithium ion batteries; even without or with a small proportion of filler, a highly porous membrane which is suitable for use in lithium ion batteries can be obtained;
the small proportion of pigment (I) or the total omission thereof enables the production of transparent foils, for example solid electrolyte foils, which are highly suitable for use in electrochromic windows;
the photocrosslinking step in the production of the cast foil does not require inert-gas conditions;
the mechanical properties of the foils resulting from the composition can be controlled from hard/brittle to soft/elastic simply through the composition of the polymer (IIa);
the presence of the polymer (IIb) means that the resultant foil is thermoplastic and can be laminated directly onto the active electrodes without addition of further auxiliaries and/or at room temperature by means of pressure;
the mechanical properties of the composition are further improved compared with those having a higher proportion of pigment; the polymeric material in the composition is chemically inert and does not need to be stored in the absence of light and air.